19. Helium core flash and the tip of the red giant branch as a
primary distance indicator

In his 1930 Harvard Observatory Monograph: ``Star Clusters,''
Harlow Shapley outlined various methods for the estimation of
distances to Galactic globular clusters. Noteworthy among these
methods is the one that assumes that the apparently brightest (giant)
stars are all of the same absolute magnitude. Tested for consistency
in those cases where more than one method of distance determination
could be made (with RR Lyrae stars, for instance), and roughly
corrected for foreground contamination (choosing the magnitude of the
sixth-brightest star as a robust estimator), Shapley went on to map
out the size scale of the Galaxy as defined by the old Population II
globular cluster system. Although the exact details of his approach
are now known to be flawed, the method as applied was of sufficient
precision that a revolutionary view of the size of our Galaxy was
obtained, and the Milky Way as an island universe started to take on a
tangible reality of its own.

Before pursuing the historical path relating to the successful
application of brightest red giant stars as extragalactic
distance indicators we digress slightly in the next section to present
a set of criteria that any extragalactic distance indicator might be
judged by. It will rapidly become clear that many of these criteria
are probably mutually exclusive in a practical sense (ultra-luminous,
locally calibrated and theoretically understood), but ideals are
seldom realized in full measure in the real world. Nevertheless, we can
establish metrics for relative performance.

CATALOGUE OF GLOBULAR CLUSTERS

Figure 17.A reproduction of part of Shapley's
original 1930 table showing the first use of the brightest red giants
in globular clusters to determine distances.